The present invention relates to a sensor control device that calculates a concentration correspondence value indicating a concentration of a specific gas included in a detection target gas, a sensor control method and a computer readable recording medium storing a program for the sensor control device.
Conventionally, a gas sensor has been used which detects a concentration of a specific gas included in a detection target gas such as an exhaust gas. For example, an NOx sensor detecting nitrogen oxide (hereinafter, referred to as “NOx”) as a specific gas includes an oxygen concentration detecting cell, a first oxygen pump cell, and a second oxygen pump cell. These cells comprise of a solid electrolyte layer having oxygen ion conductivity and a pair of electrodes, respectively. The NOx sensor pumps oxygen out of a first measuring chamber by the use of the first oxygen pump cell to keep the oxygen concentration of the detection target gas in the first measuring chamber constant so that the output voltage of the oxygen concentration detecting cell is constant. The NOx sensor applies an operation voltage between the electrodes of the second oxygen pump cell to pump oxygen out of the gas (the gas of which the oxygen concentration is adjusted by the first oxygen pump cell) introduced into the second measuring chamber from the first measuring chamber by the use of the second oxygen pump cell. The NOx concentration in the detection target gas is detected based on the value of current flowing in the second oxygen pump cell by application of the operation voltage (hereinafter, a process of detecting the NOx concentration in the detection target gas is referred to as a “detection process”).
For example, when an NOx concentration in an exhaust gas discharged from an internal combustion engine of an automobile is detected using an NOx sensor, the gas present in the second measuring chamber is in a lean state similar to the atmospheric air according to the elapsed time since the previous operation of the internal combustion engine restarts up after it stops. Accordingly, in the NOx sensor, the time until the NOx concentration in the exhaust gas can be stably measured is shortened by performing a preliminary control of temporarily and rapidly pumping out oxygen present in the second measuring chamber or oxygen included in the porous electrode in contact with the second measuring chamber to change the inside of the second measuring chamber to a predetermined state with a low oxygen concentration. For example, in a sensor control device described in JP-A-2011-137806, as the preliminary control, constant current is supplied to the second oxygen pump cell for a constant time under the control condition determined for each gas sensor after gas sensors are started up to temporarily and rapidly pump out oxygen present in the second measuring chamber. In the sensor control device described in Patent JP-A-2011-137806, even when an H2O concentration of the detection target gas varies for every startup of the sensor control device or even when the gas sensor has a deviation in output characteristics, the temporal variation in concentration correspondence value calculated after the preliminary control ends can be made to substantially maintain the same pattern.